The present invention relates to a reflector for technical lighting purposes, having a surface of aluminum which is protected from physical and chemical effects by a protective layer of aluminum oxide, and relates also to its use and to a process for its manufacture.
Reflectors with brightened surfaces of high purity aluminum or AlMg alloys are known for the purpose of reflecting light in a directional or diffuse manner. In order to achieve lasting brightness, the brightened surfaces are normally protected by an organic or inorganic coating or by an oxide layer. The oxide layers may be produced by chemical or anodic oxidation. Organic coatings may be produced by paint-type coatings, powder coatings or by laminating or coating with a plastic foil. Inorganic coatings may be made by PVD (physical vapor deposition), CVD (chemical vapor deposition), enamelling or plasma coating.
A widely used practice for manufacturing reflector surfaces is the deposition of very thin, high purity PVD Al layers on glass; such layers are usually protected by a layer of e.g. PVD-Al.sub.2 O.sub.3, PVD-SiO.sub.2 or a paint-type layer. Because of the thinness of the layer, PVD-Al layers generally cannot be anodized. Deposition of PVD-Al.sub.2 O.sub.3 or PVD-SiO.sub.2 layers is however expensive and, because of the homogeneity required to achieve good reflecting properties, the deposition of paint-type layers is complicated. Furthermore, paint-type layers generally exhibit only modest mechanical properties such as resistance to scratching, and often poor stability with respect to UV-radiation.
Another protective layer often used today for reflector surfaces is made by anodic oxidation utilizing direct current in a sulphuric acid electrolyte. The resultant protective layer exhibits a uniform layer thickness but, as a result of the process itself, exhibits high porosity. Anodic oxidation in sulphuric acid electrolytes is normally called a dc process. To achieve sufficient reflectivity using that method, the aluminum surfaces that are to serve as the reflector surfaces are normally brightened chemically or electrolytically, and subsequently protected by a transparent protective layer e.g. by a dc process. The concentration of sulphuric acid in the dc process is typically 20 wt %, the electrolyte temperature 15.degree. to 30.degree. C., the applied voltage 12 to 30V and the current density 1 to 3 A/dm.sup.2. The thickness of layer achieved is typically 1 to 10 .mu.m; the layers obtained are colorless to yellowish.
The oxide layer produced by the dc process is generally comprised of two layers viz., a pore-free, very thin base or barrier layer and a porous outer layer. The pores are produced as a result of the oxide layer being partially re-dissolved, chemically, at the surface exposed to the electrolyte. The total thickness of the oxide skin reaches its upper limit when growth and dissolution are balanced, which depends on the composition of the electrolyte, the current density and the temperature of the electrolyte.
In order to achieve adequate protection from corrosion, the porous layers produced by the dc process have to be sealed. This is normally carried out using boiling water (&gt;96.degree. C.) or water vapor (&gt;98.degree. C.). During this hydrothermal sealing the aluminum oxide swells as a result of absorbing water and the pores are closed. In the process a part of the aluminum oxide is transformed to aluminum monohydrate.
On sealing in boiling water or steam, however, often an undesired, tightly adherent sealing deposit (so called smut) is formed. As a result of atmospheric effects this smut leads to disturbing deposits which are matt to iridescent and lead to interference colors. For that reason the sealing deposits have to be removed by abrasive means. One possibility for preventing such sealing deposits is to employ special sealing baths.
The only anodic oxide layers produced in sulphuric acid that are colorless and clear are those produced on high purity aluminum and AlMg or AlMgSi alloys with high purity aluminum (&gt; or=99.85 wt % Al). In most construction alloys, as a result of heterogeneous precipitates present in the structure, more or less cloudy oxide layers are formed. Also, in most alloys, if the heat treatment is unfavorable, precipitation occurs in the structure leading to grey discoloration such as e.g. spots due to local thermal effects.
In the case of most protective surface layers produced on aluminum using the dc process, said layers for reflectors typically being 1 to 10 .mu.m thick, and in particular in the case of less pure materials such as e.g. Al 99.85, Al 99.8 or Al 99.5, alloying elements such as e.g. Fe-rich or Si-rich intermetallic phases may be incorporated in the oxide layer leading to undesired absorption or scattering of light i.e. the light is reflected at various angles. As a result, the technical characteristics of reflected light after the brightening treatment i.e. values such as e.g. the total reflectivity or the directional reflectivity, are influenced in a detrimental manner.
Due to the large thickness of the oxide layers produced by a and integral to, the dc process, the reflectivity of the surface is reduced by the absorption and scattering of light. Finally, the oxide layer in the normal thickness range of 1 to 3 .mu.m often exhibits disturbing interference effects, so called iridescence.